Canine distemper virus (CDV) infection with resulting demyelinating encephalomyelitis in dogs has been used as an animal model of subacute sclerosing panencephalitis and possibly other demyelinating diseases of man, such as multiple sclerosis. In vivo studies correlated presence of interferon in cerebrospinal fluid with CDV persistence and pathological lesions in the CNS. Only a transient appearance of interferon in serum of dogs was found after infection. Cell-mediated immune responses were greatly reduced in dogs with CDV persistence in the CNS. They were absent in dogs that died with acute CDV encephalomyelitis. In vitro studies were made, in collaboration with B. Bloom, with Ts CDV mutants in human lymphoblastoid cell lines. Pathogenicity studies with these Ts mutants in dogs are pending.